1. Field of the Invention
The present invention relates to processes for producing highly water absorptive resins having excellent water absorbency and saline solution absorbency. More particularly, it relates to processes for producing highly water absorptive resins having a high water absorption rate and water absorptive capacity, and an excellent gel-strength.
2. Description of the Prior Art
Various water absorbing materials such as sponge, pulp, paper and the like, are well known in the art. Also various synthetic products made by graft polymerization of materials containing a hydrophilic group, such as --OH, --NH.sub.2, --COOH, and the like, have been used conventionally. However, such water absorbing materials, e.g. a sponge, a pulp, and a paper, are characterized with physical mechanisms for absorbing water so that such materials have defects in that the majority of the absorbed water can be easily squeezed out by the application of external pressure. In recent years, in order to solve these defects, synthetic water-absorptive products having particular physical and chemical characteristics have been developed. The majority of such synthetic products are crosslinked polyacrylic acid salts, polymethacrylic acid salts, crosslinked polyacrylic acid-methacrylic acid copolymer salts, crosslinked saponification products of starch-acrylonitrile graft copolymer, and cellulose and acrylate-grafted copolymer. Such grafted products are on the market as sanitary napkins, sanitary pads, and diapers in the sanitary field, water absorbing containers in the civil and gardening field, and anti-dewdrop agents in the construction field.
However, although saponificated products of starch-acrylonitrile graft copolymer have a relatively high deionized water-absorbency, the saline-solution thereof is poor. Also, long-term storage is impossible due to the main component being starch.
On the other hand, partially crosslinked polyacrylic acid salts have high water absorption rates and capacity in saline solution as well as in deionized water, and they can be stored for a long period of time. Particularly, commercially available alkali metal acrylates are used as a starting material to prepare a water absorptive resin in the present invention.
Various processes are known for polymerizing acrylic acid and alkali acrylate, including bulk polymerization, aqueous solution polymerization, spray polymerization, inverse emulsion polymerization, inverse suspension polymerization, and the like. With the exception of inverse emulsion polymerization and inverse suspension polymerization, it is difficult to remove the heat of polymerization, and the viscosity of the polymerization mixture becomes too high to carry out general polymer production. Moreover, according to these processes it is difficult to obtain particulate polymers.
An example of the inverse emulsion polymerization process is disclosed in U.S. Pat. No. 3,284,393. When, for example, acrylic acid is used as the starting material, the obtained polymer is insoluble in water and does not have such an absorbency that the polymer can be called a water absorptive resin even if it is neutralized with an alkali such as sodium hydroxide or the like.
As a process for producing an acrylic acid-alkali metal acrylate polymer having a water absorbency, the inverse suspension polymerization process is mentioned in Japanese Patent Publication No. 79-30710. According to this process, a water absorptive polymer is prepared through a stable reaction by the use of a sorbitan fatty acid ester having a HLB (Hydrophilic-Lipophilic Balance) value of 3.about.6. The resin thus obtained has a high deionized water absorbency corresponding to 400.about.500 times its own weight; however, the saline solution-absorbency of said water absorptive resin is as low as 35.about.50 times its own weight.
In U.S. Pat. No. 4,497,930, there is disclosed a method for producing a water absorbent polymer comprising the steps of subjecting acrylic acid and sodium acrylate to polymerization in the presence of a sorbitan fatty acid ester having a HLB value of 3.about.6 as a dispersing agent, and adding a crosslinking agent with stirring in methanol to crosslink the surface of the resulting polymer. However, a disadvantage is that the water absorptive resin prepared by this method is unsatisfactory in water absorptive capacity because the polymerization reaction is carried out at temperatures too low (70.degree.-75.degree. C.) to stabilize the reaction.
In general, a highly water absorptive resin requires good water absorption rate, water absorptive capacity and gel-strength. These characteristics exhibit mutually opposing correlations with each other; therefore, some characteristics are partially sacrificed in order to improve other characteristics.
The foregoing three properties, namely the water absorptive capacity, the water absorption rate and the gel-strength in the water absorbent resin, are influenced by many factors such as a polymer particle's size and shape, the kinds of crosslinking agent and dispersing agent used, CMC (Critical Micelle Concentration), polymerization temperature, and so on.
In the inverse emulsion polymerization and the inverse suspension polymerization W/O systems, it is known that the use of a surfactant having a HLB value of 3.about.6 produces a stable system. But according to these polymerization reactions, the resultant particles are too minute and have many hydrophobic groups in their surface, such that the particulate polymer is swollen too slowly when contacted with water. Consequently, the water absorption rate and the water absorptive capacity become deteriorated. To solve the foregoing problem, a surfactant having a HLB value of 8.about.12, which is known to be unstable in the inverse emulsion and suspension polymerization systems, may be used. However, another problem, reaction control, arises due to agglomeration.
On the other hand, U.S. Pat. No. 4,340,706 discloses a water absorbent resin suitable for usages which require stability in the fluid-absorbed state for long periods of time. However, even the resin obtained by this method is insufficient in its rate of water absorption due to a lack of surface-crosslinking. Furthermore, in cases where the resin is prepared at low temperatures (55.degree..about.60.degree. C.) using this method, undesired residual monomers are obtained in large quantities.
Representative methods for improving the water absorption rate include increasing the surface area of the resin (i.e. decreasing the apparent specific gravity), crosslinking the surface of the particulate polymer after polymerization, or adding inorganic materials. Among the foregoing methods, the method of adding inorganic materials such as silica is advantageous. The dispersing rate of particulate polymer is remarkably improved and agglomeration of polymer particles is prevented. However, there is a disadvantage in that it is difficult to obtain the desired water absorption rate due to a poor sedimentation rate of the particulate polymer. When using the method of crosslinking the surface of the particulate polymer to improve the water absorption rate, there are also disadvantages. Minute particles which are formed tend to blow off and to agglomerate together, making it difficult to obtain a satisfactory water absorption rate.